In JavaScript, there is a built-in `seal()` method that belongs to the `Object` constructor. The `seal()` method is used to prevent new properties from being added to an object and marks existing properties as non-configurable. When you call `Object.seal(obj)`, it seals the specified `obj` object, which means you cannot add or remove properties from it. However, you can still modify the values of existing properties. Here's an example to illustrate the usage of the `seal()` method:

const obj = {
  prop1: 42,
  prop2: 'Hello'


// Attempting to add a new property
obj.prop3 = true; // Ignored, property will not be added

// Attempting to delete an existing property
delete obj.prop1; // Ignored, property will not be deleted

// Modifying the value of existing properties is allowed
obj.prop1 = 100;
obj.prop2 = 'World';

console.log(obj); // { prop1: 100, prop2: 'World' }

In this example, the `Object.seal(obj)` statement seals the `obj` object. The subsequent attempts to add or delete properties (`prop3` and `prop1`, respectively) are ignored. However, modifying the values of existing properties (`prop1` and `prop2`) is still allowed. Please note that sealing an object using `Object.seal()` does not make the object's properties read-only. You can still change their values. It only prevents structural changes to the object itself, such as adding or removing properties or changing their configurability.